It is often required to electrically sense, monitor and control an industrial process or to electrically operate equipment within a hazardous environment, for example where there are inflammable gases or vapors. This is particularly the case in the petroleum and chemical industry.
In order to overcome this problem, the general practice has been to provide an intrinsic saftey barrier between the safe area in which electrical energy is provided, electrical measurements and controls are monitored, and the hazardous area requiring actual measuring and processing in situ or remotely.
An intrinsic safety barrier essentially consists in circuitry interposed between two opposite pairs of poles for diverting any excessive current or voltage back to the poles on the safe area side away from the poles facing the hazardous area. Any faulty operation or defective component of the circuitry establishes a safe current diversion or voltage limitation to automatically keep the poles connected to circuitry within the hazardous area under safe levels of voltage and current. As a precaution against tampering or involuntary human intervention, the circuit is totally encapsulated or potted, leaving only the external connectors well identified for insertion at the interface between the safe and the hazardous area. Examples of intrinsic safety barriers of the prior art can be found in U.S. Pat. Nos. 3,527,985 of K. J. Brown; 3,684,924 of Edward J. Miller; 3,845,356 of Alvan H. Bullard et al; 3,878,434 of Ernst W. Voorhoeve; 3,813,578 of Floyd L. Tiffany; and 3,973,170 of James A. Hogan.
As a precautionary measure against overcurrent which could damage essential components of the intrinsic safety barrier circuitry, thus avoiding additional derivative circuits therein against such faulty components, at least one fuse has been provided in the past which is made part of the overall circuit. This is shown by the aforementioned Voorhoeve and Tiffany patents. However, with an encapsulated module, should the fuse be blown out, the circuit becomes definitely interrupted and the whole module must be replaced.
The prior art shows instances where one tries to do away with a fuse by automatically circumventing the faulty components. This is the solution adopted for a fuseless inverter described in U.S. Pat. No. 3,887,860 of Thomas J. Bernhardt et al. This solution is expensive and self-defeating when applied to an intrinsic safety barrier where a fuse is called for.
The present invention resides in a fuseless intrinsic safety barrier encapsulated module.